1. Field of the Invention
The present invention relates generally to a method used to diagnose imperfections in an illuminator of a lithographic tool, and more specifically to a method of diagnosing imperfections in an illuminator of a lithographic tool, as well as the impact of such imperfections upon imaging performance, using linear fitting of pupilgrams to basis functions and several novel analysis techniques.
2. Background Description
Continuous improvement in the performance of semiconductor lithographic printing tools, e.g., scanners and steppers, is a goal of all tool manufacturers. The list of suspects that contribute to less-than-perfect lithographic tool performance is lengthy. One of the most frequent contributors is the lens itself, closely followed by the tool's focusing system and stages. By way of example, the lens may include aberrations that cause errors including CD non-uniformity and linewidth abnormality (LWA) while tool focusing errors reduce CD performance and stage synchronization errors can reduce image contrast.
A less-frequently suspected cause of imperfect performance is the lithographic illuminator. This is most frequently mentioned in the context of providing (or failing to provide) isotropic illumination along the lighted slot of a scanner. This is because resist CD changes with dose, i.e., non-uniformity in the power delivered to the slot can be a significant contributor to CD non-uniformity. For this reason, scanner manufacturers provide mechanisms for both measuring and adjusting power uniformity.
In addition to providing uniform illumination power, though, the illuminator is also required to deliver a certain angular intensity distribution of light impinging on the reticle. It should be readily known by those of ordinary skill in the art that this angular intensity distribution is commonly referred to as the “pupil fill” of the illuminator. The illuminator needs typically to deliver several choices of pupil fill such as, for example, circular, annular, quadrupole and the like. But, in any option, clearly, the pupil fill should be uniform along the slot, and one would expect the actual pupil fill pattern to closely resemble the ideal case seen in textbooks or used in CD modeling calculations. This has received some attention in recent years, with work being published concerning quantification of condenser errors, the estimated effects of pupil fill errors, trial limits on pupil fill aberrations, and a means of measuring the pupil fill through generation of an intensity plot called a “pupilgram”
While these have all been valuable, little work has been done to determine the importance of pupil-fill errors relative to the other imperfections in a scanner, and even less has been done to assist in fixing the errors. For example, once a user has a set of pupilgrams, it may be important to know if the imperfection observed in the pupilgram is an important or simply a minor effect. Additionally, it would be beneficial to know how stable the pupil-fill performance of the scanner is during use. After all, in the normal course of events, lenses are replaced, optics are adjusted, and lasers are serviced. A system should be stable in the presence of these adjustments.
Prior art systems have described careful production of pupilgrams as a characterization of the illuminator. Prior art systems, however, do not take into account coupling to lithographic effects, do not analyze data with lithographic effects in mind, and do not analyze data with attention paid to trouble-shooting the illuminator. Accordingly, prior art systems are not capable of adjusting and maintaining illuminators in lithographic tools quickly and accurately, improving lithographic performance and system uptime.